This relates to the compensation of divider circuits for changes in resistance due to one or more environmental factors. Because the use of such circuits with strain gauges is of primary interest, the invention will be described in terms of such application. In will be recognized, however, that the principles disclosed may have application to other circuits.
A strain gauge is typically used by bonding it to a flexible object and measuring the change in voltage across the gauge or the change in gauge resistance as different loads are applied to the object. It is particularly advantageous to use a Wheatstone bridge in which two strain gauges are connected in series on one side of the bridge and two resistors are connected in series on the other side. Each of these four elements is in a separate diagonal of the bridge with the supply voltage applied to the nodes between the two arms and the output voltage measured between the node between the two resistors and the node between the two strain gauges. If the gauges are mounted on opposite sides of the object so that bending of the object applies tensile loading to one gauge and compressive loading to the other, the ratio of the resistance of the two strain gauges is a function of the amount of deflection in the object. Hence, the output voltage can be related to the amount of deflection in the object.
As is well known, strain gauges have both a temperature coefficient of resistance and a temperature coefficient of gauge factor or sensitivity. Thus, both their resistance and their rate of change of resistance with applied stress vary with temperature. Strain gauges can be made so that these temperature coefficients in different devices are approximately the same. However, when the gauges are bonded to an object, certain uncontrollable temperature induced strains are created that modify the temperature coefficients of resistance and sensitivity of the gauges. As a result, the voltage output from the bridge is a function of temperature.
Typically, this variation in output voltage because of changes in resistance with temperature is compensated by measuring the resistance of the gauges under zero stress at two temperatures and selecting a series/parallel network of resistance for one gauge which offsets the effects of its temperature coefficient of resistance enough that the ratio of the resistance in the two strain gauge diagonals at the two compensation temperatures is identical. This process is called temperature compensation or constant value compensation hereinafter. While this temperature compensation does improve the performance of the circuit as a measuring device, it does not guarantee that the resistance ratios are the same at any other temperature because of the complex effects of the temperature induced strain in the gauges. Moreover, no correction is made by this temperature compensation process for the variation in output voltage because of change in sensitivity with temperature.
The variation in output voltage because of change in sensitivity with temperature may be compensated by introducing a resistor in series with the bridge. The value of this resistor is selected to balance the temperature coefficient of sensitivity. More particularly, once the bridge is temperature compensated at its two compensation temperatures, its output voltage is measured at these two temperatures with maximum deflection being applied to the object on which the gauges are mounted. The series resistor is then selected so that the output voltage under this condition is the same at both compensation temperatures. This process is called span compensation.